Wyoming comment letter, September 2008

Region8

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 8 1595 Wynkoop Street DENVER, CO 80202-1129 Phone [phone] http://www.epa.gov/region08 Ref: 8EPR-EP Mr. Dennis M. Boal, Chair Wyoming Environmental Quality Council Herschler Building, Room 1714 th122 W. 25 Street Cheyenne, Wyoming 82002 Subject: EPA Action on Revisions to the Water Quality Rules and Regulations - Chapter 1, Wyoming Surface Water Quality Standards Dear Mr. Boal: The purpose of this letter is to notify you of the status of the U.S. Environmental Protection Agency Region 8 (EPA) review of the recent revisions to the Water Quality Rules and Regulations - Chapter 1, Wyoming Surface Water Quality Standards. These revisions were adopted by the Wyoming Environmental Quality Council (Council) on February 16, 2007 and submitted to EPA for review with a letter dated July 27, 2007 from John Corra, Director of the Department of Environmental Quality (DEQ). The submittal package included a statement of principal reasons and the adopted version of Chapter 1, Table A and Table B, and Implementation Policies. However, the submittal package was not complete until September 19, 2007 when EPA received certification from the Attorney General that the regulations were duly adopted pursuant to State law. Receipt of the Attorney General certification on September 19, 2007 initiated EPA’s review pursuant to Section 303(c) of the Clean Water Act (CWA or the Act) and the ...

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 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 8 1595 Wynkoop Street DENVER, CO 80202-1129 Phone [phone] http://www.epa.gov/region08 

Subject: EPA Action on Revisions to theWater Quality Rules and Regulations - Chapter 1, Wyoming Surface Water Quality Standards  

 

  Ref: 8EPR-EP   Mr. Dennis M. Boal, Chair Wyoming Environmental Quality Council Herschler Building, Room 1714 122 W. 25 th Street Cheyenne, Wyoming 82002     Dear Mr. Boal:   The purpose of this letter is to notify you of the status of the U.S. Environmental Protection Agency Region 8 (EPA) review of the recent revisions to theWater Quality Rules and Regulations - Chapter 1, Wyoming Surface Water Quality Standards. These revisions were adopted by the Wyoming Environmental Quality Council (Council) on February 16, 2007 and submitted to EPA for review with a letter dated July 27, 2007 from John Corra, Director of the Department of Environmental Quality (DEQ). The submittal package included a statement of principal reasons and the adopted version of Chapter 1, Table A and Table B, and Implementation Policies. However, the submittal package was not complete until September 19, 2007 when EPA received certification from the Attorney General that the regulations were duly adopted pursuant to State law. Receipt of the Attorney General certification on September 19, 2007 initiated EPA’s review pursuant to Section 303(c) of the Clean Water Act (CWA or the Act) and the implementing federal water quality standards regulation (40 CFR Part 131).   EPA’s review of these revisions is complete, with the exception of the following new provisions of Chapter 1, which relate to the implementation of requirements for effluent dependent waters designated for the 2D and 3D use classifications:   •   Section 2(b)(xiii), the definition of “effluent dependent water;” •   Section 2(b)(xxxviii), the definition of “net environmental benefit;” •   Section 4(b)(v), the Class 2D use subcategory; •   Section 4(c)(iv), the Class 3D use subcategory; •   The provisions of Sections 4(e), 18, 22(b), and 24 that relate specifically to the Class 2D and/or 3D use classifications; •   human health criteria provisions for Class 2D uses in Section 18;The •   Revisions to Sections 25(d), 33(b), and 34(b);

  •   Section 36, entitled “Effluent Dependent Criteria”; and •   The provisions of Appendix A(b)(ii)(3) that relate specifically to the Class 2D and 3D use classifications.   EPA’s review of these new provisions, and the supporting information outlining the implementation of these provisions provided in the document,Implementation Policies for Antidegradation, Mixing Zones, Turbidity, Use Attainability Analysis, is nearing completion. We estimate that our review of the provisions which relate to the requirements for implementation of the Class 2D and 3D use classifications will be complete in 30 days. Therefore, the actions EPA is taking today on the remainder of the revisions to Wyoming water quality standards do not address the above provisions.   The Region commends the Environmental Quality Council and the Department of Environmental Quality for adopting significant improvements to the State’s water quality standards. The principal revisions include:   •   a change in the primary bacterial indicator organism, from fecal coliform toE. coli, for protection of recreational uses and the creation of subcategories of recreational uses; •   updates to the numeric criteria in Appendix B to be consistent with EPA’s 304(a) recommendations;   •   development of site-specific criteria for chloride and selenium; and •   modifications to the statewide numeric criteria for chloride and aluminum.     Clean Water Act Review Requirements     The Clean Water Act, Section 303(c)(2), requires States and authorized Indian Tribes 1 to submit new or revised water quality standards to EPA for review. EPA is to review and approve or disapprove the submitted standards. Pursuant to CWA Section 303(c)(3), if EPA determines that any standard is not consistent with the applicable requirements of the Act, the Agency shall, not later than the ninetieth day after the date of submission, notify the State or authorized Tribe and specify the changes to meet the requirements. If such changes are not adopted by the State or authorized Tribe within ninety days after the date of notification, EPA shall promulgate the needed standard pursuant to CWA Section 303(c)(4). 2 The Region’s goal has been, and will continue to be, to work closely with States and authorized Tribes throughout the standards revision process as a means to avoid the need for such disapproval and promulgation actions.                                                     1   as States for purposes of CWA Section 303.CWA Section 518(e) specifically authorizes EPA to treat Indian tribes 2   that EPA shall promulgate standards that replaceAlthough the provisions of CWA Section 303(c) state disapproved state-adopted standards, pursuant to EPA’s Alaska Rule (40 CFR § 131.21(c)), new or revised state standards submitted to EPA after May 30, 2000 are not effective for CWA purposes until approved by EPA. See 65 FR 24641-24653. Where EPA disapproves a state’s action to revise a CWA-effective standard which the Agency has determined is consistent with all CWA requirements, no further federal action is required under CWA Section 303(c) authorities.  2  

 

Today’s Action    I am pleased to inform you that today, with certain exceptions, EPA is approving most revisions to theWater Quality Rules and Regulations - Chapter 1, Wyoming Surface Water Quality Standards.Other than the provisions listed above that relate to the State’s implementation of the 2D and 3D use classifications, the exceptions are provisions that: (1) change the designated use of a large number of waters statewide from primary contact recreation to secondary contact recreation without the required Use Attainability Analysis (UAA); (2) allow for temporary or permanent variances from the newE. colistandard outside the water quality standards-setting process; (3) include typographical errors in the thallium and toxaphene human health criteria that make them less stringent that EPA’s recommended 304(a) criteria; and (4) EPA has determined are not water quality standards. The enclosure contains the detailed rationale for today’s action.   Endangered Species Act Requirements      It is important to note that EPA’s approval of Wyoming’s water quality standards is considered a federal action which may be subject to the Section 7(a)(2) consultation requirements of the Endangered Species Act (ESA). 3 Section 7(a)(2) of the ESA states that “each federal agency … shall …insure that any action authorized, funded or carried out by such agency is not likely to jeopardize the continued existence of any endangered species or threatened species or result in the destruction or adverse modification of habitat of such species which is determined to be critical…”    EPA’s approval of the water quality standards revisions, therefore, may be subject to the results of consultation with the U.S. Fish and Wildlife Service pursuant to Section 7(a)(2) of the ESA. Nevertheless, EPA also has a CWA obligation, as a separate matter, to complete its water quality standards action. Therefore, in approving the State’s water quality standards today, EPA is completing its CWA Section 303(c) responsibilities. However, should the consultation process with the U.S. Fish and Wildlife Service identify information that supports a conclusion that one or more of these revisions is likely to jeopardize the continued existence of any endangered or threatened species or result in the destruction or adverse modification of critical habitat, EPA will revisit and amend its approval decision for those new or revised water quality standards.   New and Revised Water Quality Standards      The new or revised provisions fall into one of the following categories: (1) standards approved without condition; (2) standards approved subject to ESA consultation; (3) disapproved standards; and (4) provisions for which EPA is taking no action.                                                       3 Where EPA concludes that its approval will have no effect on listed endangered or threatened species, or is otherwise not subject to ESA consultation, EPA can issue an unconditional approval.   3    

Standards Approved Without Condition   •   Definitions (Section 2(b)) •   Class 4C Waters (Section 4(d)(iii)) •   2C Human Health Criteria (Section 18)Class •   Change toE. colias the bacterial indicator for recreational uses (Section 27(a) and (c)) •   Creation of secondary contact recreational use (Section 27(b) and Section 4(e))   Seasonal recreational uses (Section 27(b)) • •   Authority to establish site-specific criteria (Sections 33(c))   Surface Water Classifications (Appendix A(b)(ii)(3)(A)) • •Human Health Priority Pollutants (Appendix B)   •   Human Health Non-Priority Pollutants (Appendix B) •   Site-Specific Criteria (Appendix B)   Standards Approved Subject to ESA Consultation   •   Flow Conditions (Section 11(a)(i)) •   Ammonia (Section 21(a)(i)) •   Clarification of which Classes of waters are subject to the Appendix B aquatic life criteria (Section 21(b)) •   Radioactive Material (Section 22(b)) •   Dissolved Oxygen (Section 24) •   Aquatic Life Priority Pollutants (Appendix B) •   Aquatic Life Non-Priority Pollutants (Appendix B) •   Ammonia Table (Appendix C) •   Cadmium Hardness Equations (Appendix F) •   Hardness Cap Policy (Appendix F)   Disapproved Standards     •   Elements of Section 27(a) •   Section 27(d) •   Thallium and Toxaphene human health criteria    EPA is disapproving four of the State’s revised provisions: (1) elements of Section 27(a) that change the designated use of a large number of waters statewide from primary contact recreation to secondary contact recreation without the required Use Attainability Analysis (UAA); (2) Section 27(d) which allows temporary or permanent variances from the newE. coli   standard outside the water quality standards-setting process; and (3 and 4) typographical errors in the thallium and toxaphene human health criteria listed in Appendix B of Chapter 1, “Water Quality Criteria,” that make these criteria less stringent than EPA’s recommended 304(a) criteria.   In anticipation of this disapproval action, EPA and DEQ have been in discussion on the  4  

 

  general matter of available options for addressing and resolving the UAA and variance issues. The options discussed to date are identified in the enclosure of this letter. The Region will continue to work with the State to explore options that meet the State’s needs and are consistent with the Act.   Pursuant to 40 CFR § 131.21(c), new or revised state standards submitted to EPA after May 30, 2000 are not effective for CWA purposes until approved by EPA. 4 Therefore, disapproval does not change applicable standards and is not subject to ESA consultation. For a more detailed discussion of the options available to the State to address EPA’s disapproval of the four revised standards, please refer to the discussion of “Disapproved Standards” in the enclosure to this letter, which also includes information regarding the potential for EPA promulgation of replacement federal standards for each disapproved standard.   Provisions For Which EPA Is Taking No Action   Other than the provisions listed above that relate to the State’s implementation of the 2D and 3D use classifications, there are several provisions that EPA is not acting on today because EPA has determined they are not water quality standards requiring EPA review and approval under CWA Section 303(c). These provisions are listed in the enclosure and include minor revisions such as correction of references, addition of references, and deletion of duplicate provisions.   Indian Country     The water quality standards approvals in today’s letter apply only to water bodies in the State of Wyoming, and do not apply to waters that are within Indian country, as defined in 18 U.S.C. Section 1151. “Indian country” also includes any land held in trust by the United States for an Indian tribe and any other areas defined as “Indian country” within the meaning of 18 U.S.C. 1151. Today’s letter is not intended as an action to approve or disapprove water quality standards applying to waters within Indian country. EPA, or authorized Indian tribes, as appropriate, will retain responsibilities for water quality standards for waters within Indian country.   Conclusion    EPA Region 8 commends the Council and the Department for the significant improvements to Wyoming’s water quality standards. I will keep you apprised on our progress in completing the remainder of our review. If you have questions concerning this letter, the most knowledgeable person on my staff is Fritz Wagener, who can be reached at 303-312-6219.                                                         4   See EPA’s Alaska Rule (65 FR 24641-24653).    5  

     

Sincerely,

  Carol L. Campbell Assistant Regional Administrator Office of Ecosystems Protection and Remediation

  Enclosure   cc: John Corra, Director, Wyoming Department of Environmental Quality  John Wagner, Administrator, Water Quality Division, Wyoming Department of Environmental Quality  Brian Kelly, Field Supervisor, Wyoming Field Office, U.S. Fish and Wildlife Service Amy Newman, Office of Science and Technology, EPA Headquarters    

   

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7-7-08 Draft – do not distribute, internal deliberative

Enclosure   Rationale for EPA’s Action on the Revisions to Wyoming’s Water Quality Standards     This enclosure provides the rationale for today’s EPA action. The discussion below is organized into six major sections. The first section provides background on the development of a process for addressing aquatic life use designation and level of protection for ephemeral waters because this is important to understanding the next section. The second section discusses Wyoming’s creation of two new use classifications for effluent-dependent waters and a new procedure for calculating ambient-based criteria for these classifications. The remaining four sections are organized sequentially based on Sections ofChapter 1, Wyoming Surface Water Quality Standardsand the ESA consultation category for each provision: (1) standards approved without condition; (2) standards approved subject to ESA consultation; (3) disapproved standards; and (4) provisions for which EPA is taking no action. This enclosure contains the detailed rationale for these decisions.   The Wyoming water quality standards submittal package included revisions to Wyoming’s UAA Implementation Policy. The Implementation Policy was not adopted by the Council and is not a water quality standard. Nevertheless, because implementation policies/procedures affect the application of the standards and, as well, inform EPA as to a State’s interpretation of its standards, EPA’s review of State water quality standards includes review of such policies/procedures. The content of this policy played an important role in EPA’s conclusions.   Ephemeral Waters     For many years EPA and Wyoming have discussed the appropriate aquatic life use designation and level of protection for ephemeral 1 and effluent-dependent 2 waters, both common in this arid part of the country. Although Wyoming’s new use classifications are specific to effluent- dependent waters, understanding the development of a process for addressing ephemeral waters is important to understanding the effluent-dependent situation. Effluent-dependent waters occur where a discharge has altered an otherwise naturally occurring ephemeral waterbody. As such, effluent-dependent waters begin as ephemeral waters, and developing their appropriate classification and protection has included in-depth discussions of ephemeral ecosystems.   EPA acknowledges that all ephemeral waters sustain some level of aquatic life. And within the range of low flow habitat types, aquatic communities form a continuum, making it difficult, in the biological sense, to identify the threshold where an aquatic life use begins. The federal regulation allows designated use removal where low flow conditions prevent the attainment of a use (40 CFR Section 131.10(g)(2)). Therefore, states and EPA are left to reconcile the biological                                                   1 A stream that flows only in direct response to a precipitation or snowmelt event in the immediate watershed and whose channel does not intersect the ground water table.   2 An effluent-dependent waterbody is one that would be ephemeral without the presence of wastewater effluent, but which has continuous or intermittent flows for all or a portion of its reach as the result of the discharge of treated wastewater. 

7-7-08 Draft—Do not distribu te, Internal deliberative realities with the water quality standards regulations.   The Region has addressed this issue by applying a hydrologic threshold rather than a biological one. “A naturally ephemeral condition with a short hydroperiod” is the hydrologic threshold the Region has used in making a flow-based distinction between waters supporting and not supporting aquatic life uses. Where WY has demonstrated that a waterbody is: (1) naturally ephemeral with a short hydroperiod, (2) there are either no regulated discharges or the discharges are intermittent and do not alter the natural, ephemeral character of the water, and (3) a UAA demonstrates that there is no existing occurrence of an aquatic life use consistent with Section 4(d)(ii) of Wyoming’s water quality standards and that the natural, ephemeral flow conditions prevent attainment of an aquatic life use consistent with 40 CFR Section 131.10(g)(2), the Region has accepted that as a basis for either removing or not designating an aquatic life use. Wyoming’s water quality standards and UAA Implementation Policy use occurrence of wetland vegetation as an indicator of whether or not normal flow conditions are sufficient to support aquatic life. The Region concluded that this was an appropriate indicator of a short hydroperiod that would prevent attainment of an aquatic life use, but would consider other scientifically defensible indicators as well.   In situations where there is a continuous, regulated discharge to an ephemeral stream that is of sufficient volume to alter the natural, ephemeral character of the water and establish or sustain an aquatic life use, EPA’s position is that the resulting aquatic community must be protected. (63 FR 36755). This is the situation that is addressed in the creation of the new 2D and 3D use classifications.   Creation of Use Classifications 2D and 3D and the Procedure for Calculating Ambient- Based Criteria for these Classifications   Description of 2D and 3D Classifications   New Sections 4(b)(v) and 4(c)(iv) establish new categories of aquatic life uses, 2D and 3D, applicable to effluent-dependent waters. Class 2D, effluent-dependent fisheries, will be applied to waters that support resident fish populations where support of the fishery is wholly dependent upon permitted effluent discharges. Class 3D, effluent-dependent non-fish bearing waters, will be applied to waters where the support of the existing, non-fishery aquatic community is wholly dependent upon permitted effluent discharges. The designated uses for 2D are game or nongame fisheries, fish consumption, aquatic life other than fish, recreation, wildlife, industry, agriculture, and scenic value. Class 3D has the same designated uses except game or nongame fisheries and fish consumption are not included.   The aquatic life criteria in Appendix B apply to Class 2D and 3D except for: (1) parameters where ambient-based criteria are adopted pursuant to Section 36; or (2) temperature, dissolved oxygen, and ammonia, for which narrative criteria apply. EPA’s understanding is that the only difference between the criteria for 2D and 3D is that the translation of the narrative dissolved oxygen standard in Section 24 would vary based on the fish population present in 2D waters. 2    

7-7-08 Draft—Do not distribu te, Internal deliberative

  Which waters are eligible for 2D or 3D?   Authority to Establish Ambient-Based Criteria   Sections 33(b) and (c) were amended to provide authority for the Water Quality Administrator to establish ambient-based criteria on effluent-dependent waters. Section 34(b) was revised to clarify that ambient-based criteria do not need to be approved by the Environmental Quality Council and will be established according to the provisions of Section 36.   New Procedure for Calculating Ambient-Based Criteria for Class 2D and 3D Waters   The new Section 36 describes a performance-based 3 approach that relies on adoption of a process (i.e., a criterion derivation methodology) rather than a specific outcome (i.e., concentration limit for a pollutant).   If EPA approves a performance-based procedure for calculating the ambient-based criteria, EPA would not review and approve/disapprove site- specific criteria adjustments for individual waterbodies. EPA would review and approve/disapprove the UAA-based designation of Class 2D or 3D and review site-specific criteria adjustments to ensure the performance-based procedure is properly implemented. However, EPA would review and approve/disapprove category-specific criteria adopted pursuant to this procedure. In addition, the ambient-based criteria must ensure maintenance of existing water quality and would be used for all other purposes of the Act, such as NPDES permits and CWA Section 303(d) listing of impaired waters.   Section 36 provides the Water Quality Administrator the authority to make adjustments to the aquatic life criteria in Appendix B applicable to Class 2D (waters with fish) and 3D (waters with aquatic life other than fish) where four conditions can be met:   (1) the waterbody is effluent-dependent; (2) a discharge has been shown to create an environmental benefit and removal of the discharge would cause more environmental harm than leaving it in place; (3) there is a credible threat to remove the discharge; and (4) appropriate safeguards are in place, ensuring that downstream uses will be protected and the discharge will pose no health risk or hazard to humans, livestock or wildlife (in addressing the potential adverse effects to humans, livestock and wildlife, the hazard analysis would focus on persistent and bioaccumualtive toxics in the discharge).   Where these “net environmental benefit” factors are satisfied and the Appendix B criteria are  exceeded, the numeric criteria for eligible parameters will be set equal to the background condition. Section 36(b) establishes that the background condition will be based on the highest recorded concentration plus one standard deviation and clarifies the ambient-based criteria will be implemented as instantaneous maximum values. The ambient-based criteria can be calculated on either a categorical or site-specific basis. The procedure described in Section 36 is only                                                   3   See 65 FR 24648 for further discussion.    3  

7-7-08 Draft—Do not distribu te, Internal deliberative available for parameters that exceed the otherwise adopted statewide (Appendix B) criteria.   Process for Use Change to 2D or 3D and Calculation of Ambient-Based Criteria   EPA’s understanding, based on discussions with DEQ staff, is that Wyoming intends to implement a two-step process. The first step would be completion of a UAA that addresses the four net environmental benefit factors. The stream segments where the 2D and 3D classifications apply will be defined in the UAA. The State will make a UAA-based water quality standards revision where appropriate and submit the designation of Class 2D or 3D to EPA for review and approval/disapproval. The Wyoming Surface Water Classification List will be updated to include these new classifications.   The second step would be the collection of a year of data for the calculation of the ambient-based criteria. Wyoming will also maintain a publically available, comprehensive list of all ambient- based criteria decisions made using the Section 36 process.   UAAs for 2D and 3D Classifications   The State added a new Section VI to its UAA Implementation Policy. 4 Section VI documents the data requirements and decision making process that will be used to determine whether each of the four net environmental benefit conditions are satisfied. Section VI states as follows: 1.   In order to demonstrate a water is effluent-dependent, the UAA must show through a weight of evidence approach that the water is comprised of 100% permitted effluent. 2.   presumed that the water is an environmental benefit for the aquatic life thatIt is colonized it, for other wildlife that may use the water, livestock watering, irrigation, industrial uses, and non-consumptive recreation and scenic values. This presumption of environmental benefits can be overridden where the quality or condition of the water poses a threat or hazard to non-aquatic wildlife, livestock, industrial uses, or human health (see #4 below). 3.   is presumed for conventional oil and gas dischargesThe demonstration for credible threat based on two considerations: (1) alternatives to surface discharge are the norm for the industry, with an exemption from that practice allowed only west of the 98 th meridian; and (2) an economic analysis provided by EPA Headquarters showing that available treatment options are, as a general matter, more expensive than the available non- discharge options. For other types of discharges, the credible threat demonstration is made on a case-by-case or categorical basis. 4.   The hazard analysis includes an initial screening of the permitted effluent for pollutants of concern. Screening parameters will vary based on type of discharge. The majority of waters in Wyoming eligible for the 2D or 3D Classification are created by discharges from oil and gas production or mining activities. For these types of discharges, the hazard                                                   4 Implementation Policy was not adopted by the Council and is not a water quality standard. Nevertheless,The because implementation policies and procedures affect the application of the standards and, as well, inform EPA as to a state’s interpretation of its standards, EPA’s review included the new Section VI and the content of this policy played an important role in EPA’s conclusions about the acceptability of Section 36.  4  

7-7-08 Draft—Do not distribu te, Internal deliberative analysis must include selenium and mercury. a.   analysis will focus on the mortality and impaired reproduction inThe selenium birds. Where there is little or no hazard to birds, the other designated uses will be considered protected. Where the effluent concentration of selenium exceeds the statewide chronic aquatic life criterion, a whole body fish and/or macroinvertebrate tissue analysis will be conducted. If whole body tissue concentrations are≤7.9 µg/g dry weight, the water will not be considered a hazard and this criterion will be established for the water along with an ambient- based water column value. Where the whole body tissue concentrations are > 7.9 µg/g dry weight, the water will be considered a hazard and a whole body tissue criterion of 7.9 µg/g dry weight will be established for the water, but there will be no ambient-based criterion established and the water will be put on the 303(d) list. b.   For mercury, where the effluent concentration exceeds the statewide chronic aquatic life criterion and the discharge can be expected to reach a fish bearing water, whole body fish tissue analysis will be conducted. If whole body concentrations are≤0.3 mg methylmercury/kg fish the water will not be considered a hazard and this criterion will be established for the water along with an ambient-based water column value. If whole body concentrations are > 0.3 mg methylmercury/kg fish, the water will be considered a hazard and 0.3 mg methylmercury/kg fish criterion will be established for the water, but there will be no ambient-based criterion established and the water will be put on the 303(d) list. Where the effluent concentration exceeds the statewide chronic aquatic life criterion but the discharge is not expected to reach a fish bearing water, sediment anaylsis may be required. Ambient-based criteria may be established where sediment concentrations are≤23 mg/kg inorganic mercury and 26 mg/kg methylmercury. In no circumstances will the ambient-based criterion exceed 2 µg/L total recoverable mercury.   Calculation of Ambient-Based Criteria   Section VI also describes the process for calculating ambient-based criteria for eligible parameters. The background concentration is the highest concentration recorded over the course of a one year period with a minimum data set of at least 12 samples. Samples may be collected either at the discharge outfall or from a representative point in the stream downstream from the permitted outfall, consistent with State guidance. The margin of error is one standard deviation calculated from the same data set used to establish background.   EPA’s understanding, based on discussions with DEQ staff, is that Wyoming intends that new discharges to a stream segment where ambient-based criteria have been calculated according to Section 36 will be required to meet the same ambient-based criteria as the existing discharger (i.e., new ambient-based criteria would not be calculated for each new discharge).  

   

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